A device for determining the loading surface of a shaft furnace using a radar probe is described in the document EP-Al-0,291,757. In this type of device it is recommended to dispose the electronic signal transmitting and receiving circuits outside the furnace in order to shield them from the severe conditions prevailing inside. At the same time, it is necessary to provide antennas inside the furnace in order to view the loading surface directly. It is therefore necessary to provide, one or two transmission lines to connect the electronic radar circuits to the antennas. At the high microwave frequencies which are necessary in order to obtain a very narrow beam from the antenna of limited dimensions, the losses associated with the transmission lines must be reduced as much as possible. This is the reason why waveguides are generally used as transmission lines since these guides have lower losses at high frequencies than coaxial cables which can also contain dielectric materials which are not suitable for the high temperatures inside a shaft frequency.
However, the use of relatively long waveguides, which must represent the major portion of the distance between the electronic circuits and the antennas, can cause measurement problems or can be at the origin of error sources. These problems derive, on the one hand, from thermal expansions, and from distortions and flexions of the arm of the probe which is cantilevered above the loading surface and, on the other hand, from the electromagnetic propagation effect more generally known as frequency dispersion and which is produced as a consequence of variations in the speed of the signal in the waveguide as a function of frequency. It is necessary to recall that the identification of the signal reflected by the loading surface is carried out, in the electronic circuit outside the furnace, by comparison with a reference signal generated by this electronic circuit. Now, if the signal undergoes modifications in the transmission line, whether in the forward or return direction, the reflected signal is difficult to identify because it is no longer correlated with the reference signal and there is risk of measurement error or of generating a false measurement signal. This problem arises both for monostatic antennas which have only one antenna serving as a transmitter and a receiver and only one waveguide for for the transmitted and the reflected signals, and for bistatic antennas, i.e., the system when has a transmitting antenna and a separate receiving antenna, each associated with a separate waveguide.